Scientists estimate that 99% of the harm caused by smoking stems from combustion and tar.

Nicotine, which is both addictive and a stimulant, is not 100% safe, but the risk is similar to caffeine, and is vastly safer when taken in a noncombustible form.
(Source: ASH UK “…the harm from smoking is caused almost exclusively by toxins present in tobacco released through combustion” http://www.ash.org.uk/files/documents/ASH_715.pdf)

In contrast to electronic cigarettes, nicotine is vaporized by heating up a liquid base. Crucially, at no point does combustion take place. The UK Government Nudge Unit believes devices like ecigarettes have the ability to save thousands of lives.
(source: http://www.guardian.co.uk/society/2011/sep/14/smokeless-nicotine-cigarettes-government.)

The principle ingredient in electronic cigarette cartridges and ‘eliquid’ is propylene glycol. Propylene glycol (which is no longer used in the blu electronic cigarette brand) is regarded as very safe, and has been used in air-conditioners, medicines, drinking water and to sterilize children’s hospital wards. Some people are, however, allergic to propylene glycol, and an alternative for these people is to use a vegetable based glycerine.
(source: Safety Report by Health New Zealand http://www.healthnz.co.nz/RuyanCartridgeReport30-Oct-08.pdf)

Studies Regarding Electronic Cigarettes

There have been a number of studies done on electronic cigarettes. Following is a small selection:

Flouris Et Al 2012: Acute effects of electronic and tobacco cigarette smoking on complete blood count(Source: http://www.sciencedirect.com/science/article/pii/S0278691512005030
Found that vaping, in contrast to smoking, has no effect on the number of white blood cell, lymphocyte, and granulocyte counts in blood.)

Polosa et al (Italian Non Smoking Association): The effect of electronic cigarettes in helping cigarettes smokers to reduce or eliminate tobacco
Found that e-cigarettes helped over 50% of smokers to reduce or eliminate tobacco cigarettes. The doctor discussed the results here: http://www.ecigarettedirect.co.uk/ashtray-blog/2011/10/electronic-cigarette-interview-professor-riccardo-polosa.html.

Cahn and Siegel; 2011 Electronic cigarettes as a harm reduction strategy for tobacco control: A step forward or a repeat of past mistakes?
(Source: http://www.palgrave-journals.com/jphp/journal/v32/n1/abs/jphp201041a.html Found that electronic cigarettes are “undoubtedly” safer than tobacco cigarettes.)

Quotes From Scientists and Campaigners:

Dr Joel Nitzkin, Chair of the Tobacco Control Task Force for the American Association of Public Health Physicians: “…we have every reason to believe that the hazard posed by e-cigarettes would be much lower than one percent of the hazard posed by cigarettes.”

Professor Michael Siegel, associated Professor at the Boston University of Public Health: “Inhaling nicotine cannot be nearly as dangerous as inhaling nicotine plus thousands of other chemicals, including more than 40 carcinogens.”

Professor Carl Phillips, former Professor at the University of Alberta, and researcher in the field of tobacco harm reduction: “I think there’s absolutely no doubt that it [the e-cigarette] is a safer alternative to regular cigarettes.”

David Sweanor, Former adviser to the WHO on tobacco control: “If there is anyone who believes cigarettes are no more hazardous than e-cigarettes, I’d recommend a remedial course in basic sciences.”

Dr Murray Laugesen, Former principal medical officer in the New Zealand department of health and winner of a WHO medal for work in the field of tobacco control: E-Cigarettes are “harmless: inhaled or exhaled.”

Further Information:

Action on Smoking and Health: Action on Smoking and Health Briefing on Electronic Cigarettes: http://www.ash.org.uk/files/documents/ASH_715.pdf

Europe making bad policy on nicotine – five ways to make it better: http://www.clivebates.com/?p=697 (By Clive Bates, former direction of ASH UK)